This invention relates, in general, to blades for a turbomachine such as a gas turbine; and, in particular, to cooling such blades at their tip portions.
A turbomachine such as a gas turbine, includes a turbine having a hot gas path comprising alternate annular stages of stationary nozzles and rotating blades. The blades are affixed to a disk which is, in turn, fixed to a rotor so that as hot gas flows in a generally axial direction through the hot gas path it will cause the transfer of kinetic energy to the blades and disk, thereby causing the rotor to be turned. The hot gas is released from an upstream combustion reaction and may have a temperature on the order of 2000 degrees Fahrenheit or higher. These elevated temperatures are typically accommodated by the cooling of stationary and rotating components in the hot gas path.
One method of cooling rotating turbine blades is to duct compressor discharge air axially along the gas turbine rotor until it can be picked up by the rotating blade to be cooled. The blade is formed with an interior cavity so that the cooling air is sent radially through the blade and then is discharged from the blade into the hot gas path through blade surface holes. The hot gas path includes an annular, radially outward shroud which extends axially and surrounds a rotating bladed stage so that the radial clearance between the shroud and the blade tips is as small as possible so as to minimize axial leakage of hot gas therebetween. If gas is permitted to bypass a bladed stage, it adversely impacts on turbine efficiency. Of course, the aforesaid radial clearance is also adjusted for avoiding the blade tips rubbing against the outer shroud.
Some blade tips are formed by joining radially extending sidewalls and radial holes are drilled through the tip into the interior cavity to allow cooling air to be removed from the interior cavity. However, some blades are not thick enough at their tips to permit such drilling; and if such blades were thick enough then it might be expected that an accidental rub between the blade and the shroud could cause undesirable effects upon the shroud. Even more significant, the use of a small radial clearance between the shroud and the blade tip could cause such radial drilled holes to be impeded from achieving a sufficient flow volume of discharged cooling air; or conversely, a larger radial clearance sufficient to permit adequate discharge of cooling air flow would result in unacceptable hot gas losses therethrough.
One solution to the radial tip clearance dilemma, is found in the discovery of a blade tip cap which is recessed from the tip of the blade to create and define an open plenum at the blade tip. The plenum is further defined by extensions of the opposite blade sidewalls. Cooling air, exhausted from the blade interior cavity, is fed into the plenum through at least one hole which connects the blade interior cavity with the plenum. The depth of the plenum; or conversely, the height of the sidewall extensions is dependent upon the cooling requirements. For example, the more cooling air to be removed from the blade interior cavity, the deeper the plenum or conversely the higher the plenum walls. However, as the height of the plenum walls is increased it becomes more difficult to cool because tip areas are further removed from cooled blade portions thereby increasing the length of the conduction path. This problem is especially acute at the leading edge of the turbine blade.
This problem was recognized in U.S. Pat. No. 4,142,824, issued to inventor Richard H. Andersen issued Mar. 6, 1979 and assigned to the assignee of the present invention. This patent is incorporated herein by reference. The patent teaches that certain external surfaces of the turbine blade may be cooled through conduction by means of passageways either drilled within the blade or formed by means of sleeves fastened to the outer circumference of the blades. This solution to the problem adds to the cost of manufacture while also being limited to situations where the blade design allows the drilling of interior passages or the application of cooling sleeves.
It is therefore an object of the present invention to provide an improved blade design having improved blade tip cooling.
It is another object of the invention to provide a blade tip design which will minimize the required height of sidewall extensions.
It is yet another object of the invention to provide a blade tip design which will accommodate blade tip cooling requirements independent of radial clearance requirements with respect to the surrounding shroud.